The present invention relates to pictorial display devices such as television devices and, more particularly, to a method and apparatus for improving contrast ratio and light output of projection type display devices.
Projection type pictorial display devices generally are classified as rear or back projection screens and front projection screens. With back projection screens, an image is displayed upon the selectively darkened front face of the screen by projection onto the back face of the screen. In front projection devices, an image is projected directly onto the front face of a screen from the front face. Back projection systems are generally preferable if space is available behind the screen since the contrast ratio with high ambient light levels is higher. Front projection screens are used in other applications in which the projectors must be placed forward and spaced from the screen, such as, for example, in systems used for passenger viewing in large commercial aircraft. However, in any type of projection system the primary concern is with how to improve contrast and picture quality in the presence of ambient lighting.
U.S. Pat. Nos. 3,523,717 and 3,462,214 describe one form of back projection screen formed as a composite structure having refracting lens components and focusing lens components integrally formed within the surface of the screen. The focusing lens components are situated along the back face of the projection screen proximate the image to be viewed and the radius of curvature of the focusing lens components are so dimensioned relative to the thickness of the screen that the foci of the focusing lens components are located along the front face of the screen. Refracting lens components are formed in the screen in optical registry with the focusing lens components and the front face of the screen is covered with a darkened overcoating at all locations except the foci of the focusing lens components. The alignment of the refracting lens components with the focusing lens components inhibits the formation of moire while the incorporation of both lens components upon a single projection screen reduces multiple reflections between the surfaces of the aligned lenses. Because the focusing lens components forming the back face of the projection screen are small, e.g., preferably cylindrical lenses less than 50 mils in arcuate span, effective optical registration of the refracting lens components and the focusing lens components generally can be accomplished only by the formation of both the refracting lens components and the focusing lens components in a single projection screen.
It is desirable to coat the front surface of the screen with a dark or light impervious overlayer at all points except the foci of the focusing lens components to provide for the high contrast necessary for high quality pictorial displays. More particularly, by coating the front face of the screen at all locations except those locations at the foci of the focusing lens components, all light impinging on the screen outside of these foci will be absorbed by the black non-reflective areas of the screen and will not be reflected toward the viewer of the screen. In this manner, the amount of reflected light is significantly reduced and picture contrast is improved. In the disclosed invention of the above patents, the preferred method of selectively applying a dark overcoating onto the cylindrical lenses of the front face of the screen is to coat the entire front face of the projection screen with an approximately 2 mil thick layer of dyed photo resist and subsequently to etch the photo resist at the foci of the cylindrical lenses. The precision of dyed photo resist removable afforded by etching minimizes the required tolerance and the size of clear sections at the lens foci thereby darkening the overall appearance of the picture screen. In an alternate method, the black overcoating may be deposited using a mask to selectively coat the front face of the projection screen.
While the above described method has proven to be generally satisfactory in producing a back projection screen having good contrast ratio, applicants have found that the application of the photo resist layer onto the front face of the screen with the layer then being etched at the foci of the cylindrical lenses does not necessarily provide a projection screen in which the projected image is properly focused at each of the etched areas. More particularly, applicants have found that even small differences in projector location used with a particular screen will cause the light from the projector to be focused at different points. Accordingly, a projection screen which is satisfactory for one particular projector location may not produce a uniform image brightness or an image with the best contrast with a different projector location. Accordingly, it is desirable to provide a projection screen which has etched areas precisely positioned to match the particular projector location used with the screen.
Although the two above mentioned patents disclose back projection screens that minimize the distance from screen to projector, e.g., to about 0.433 times the image height, that distance is still a greater distance than people would like to have for displays that are very large. A front projection screen with a ceiling mounted projector of the type commonly used in large viewing areas (auditoriums and sports bars) or in commercial aircraft is physically much more attractive since the throw distance is in front of the screen and does not require a space behind the screen. However, prior art front projection screens have not been capable of giving good contrast in the presence of high ambient light levels. By increasing the gain of the screen, one can somewhat improve the contrast but at the loss of viewing angle. An ideal front projection screen would appear as a solid black surface in the presence of reflected light but would still reflect all of the light that falls on the screen from a projector. Accordingly, it would be desirable to have a front projection screen which minimizes reflected light from external sources while maximizing reflected light from a projector.